1. Field of the Invention
The present invention relates to a tube for use in a fixed angle centrifuge rotor.
2. Description of the Prior Art
Tubes, whether capped or uncapped, are used to carry sample liquid in centrifugation protocols. Such tubes may have indicated thereon a rated nominal capacity that defines the full volume of the liquid contained within the tube. In addition, the tube may have graduations permanently formed thereon denoting the volume within the tube when the tube is filled to various heights. Such graduated centrifuge tubes are available from a variety of vendors, with the round or conical tubes as shown at page 41 of the Nalgene.TM. Labware Manual (1991) being representative.
When a tube is used in a fixed angle rotor, i.e., a rotor in which the axis of each rotor cavity is inclined at a predetermined angle .THETA. with respect to the axis of rotation, care must be taken to avoid filling the tube to a level that could, due to meniscus reorientation, cause spillage (in an uncapped tube) or cause challenge to the tube seal (in a capped tube, if challenge is a concern).
FIG. 1 permits a full understanding of the problem of meniscus reorientation encountered when a tube as shown is first inserted into a rotor and when the tube is thereafter spun. In FIG. 1 the rotor R is assumed to have a plurality of cavities C each of which is oriented at a predetermined rotor angle .THETA., shown as forty-five degrees (45.degree. ) with respect to the axis of rotation CL. When the axis A of the tube T is oriented vertically the tube T is filled with a volume of liquid reaching the level P.sub.f. When the tube T is first inserted into the rotor the liquid level re-orients to the static position indicated by the character P.sub.s. As the rotor R is spun centrifugal force causes the liquid to reorient to the dynamic level P.sub.d. Note that in a rotor having a forty-five degree angle and a tube sized and configured as shown in FIG. 1, the volume of liquid that would prevent spillage or seal contact in the static case (the static level P.sub.s) is equal to the liquid volume that would prevent spillage or seal challenge while spinning (the dynamic level P.sub.d). If the rotor angle is less than forty-five degrees the dynamic volume that prevents spillage or seal challenge is less than the corresponding static volume. Conversely, if the rotor angle is greater than forty-five degrees the dynamic volume that prevents spillage or seal challenge is greater than the corresponding static volume.
As noted in FORD and Graham, "An Introduction to Centrifugation", page 18, Bios Scientific Publishers, Ltd. (1991), uncapped tubes should be filled only to a level consonant with the degree of such meniscus reorientation. The appropriate level for a tube of given physical parameters and of a given configuration depends the magnitude of the angle .THETA. in the centrifuge rotor. The angle of inclination may be different for each fixed angle rotor able to be spun in a given centrifuge instrument, but is typically a value less than forty-five degrees.
The problem of overfilling of tubes may be avoiding by use of a filling gauge such as that shown at page 4-10, Beckman Instruments, "Rotors and Tubes for Preparative Ultracentrifuges--An Operator's Manual", LR--IM--12, March 1985. The filling gauge includes reference to rotor types. Prior to filling, each tube must be aligned with the filling gauge and suitably marked, as with a felt-tipped pen. Such a procedure may be viewed as laborious and time consuming. Moreover, since the marking on the tube may become obliterated when the tube is washed or sterilized the marking procedure using the filling gauge must be repeated each time the tube is re-used.
In view of the foregoing it is believed advantageous to provide a tube for use in a fixed angle centrifuge rotor that includes at least one, but preferably a series, of indicator marking(s) that are permanently provided thereon whereby the tube may be quickly and conveniently filled to a volume level appropriate for use in any one of a plurality of fixed angle rotors without risk of tube spillage or seal challenge due to meniscus re-orientation.